1. Field of the Invention
This invention relates to an improved process for coating glass surfaces, particularly glass bottles, to improve their resistance to shattering.
2. Description of the Prior Art
The coating of glass surfaces, particularly glass bottles, with a transparent layer of polyurethane to improve their resistance to shattering is known and has been the subject of numerous publications (cf. German Offenlegungsschrift Nos. 2,731,776; 2,746,017 and 2,746,006).
In all these publications, the glass surfaces are coated with solvent-free two-component coating compositions. Before coating, the glass surfaces are treated with an aqueous or alcoholic silane size to improve the adhesion of the lacquer to the glass. In practice, the glass surface pretreated with the adhesion promoter is normally dried at temperatures around 100.degree. C. to enable the pretreatment to develop its full adhesion-promoting effect. In addition to the two-component polyurethane system and the usual additives and auxiliaries, the coating composition applied in the second stage of the process contains in particular catalysts which accelerate hardening of the binder, i.e. the isocyanate addition reaction. A serious practical disadvantage in this respect is that the processing time of the two-component polyurethane coating compositions is shortened to a considerable extent by the presence of the catalysts.
In addition, it is known from German Auslegeschrift No. 1,621,883 that a lacquer layer containing a catalyst which accelerates the isocyanate addition reaction may be initially applied to surfaces to be coated with solvent-containing polyurethane lacquer, the lacquer layer thus obtained subsequently being coated with a solvent-containing catalyst-free two-component polyurethane lacquer. The first lacquer layer is dissolved to an extent and some of the catalyst is carried by convection into the surface lacquer where it is activated.
It has now surprisingly been found that this principle may also be applied to solvent-free coating compositions of the type used for coating glass surfaces. It has also been found that the priming lacquer used for this purpose may also contain the known silane-based adhesion promoters and that there is no need for the priming lacquer to be stoved if, at the same time, it contains a binder based on a linear polyurethane polyurea of the type described in detail hereinafter. Another advantage of using a priming lacquer of this type lies in the increased mechanical strength of the coating as a whole because the simultaneous use of a priming lacquer containing a fully reacted polyurethane polyurea as binder and of a coating composition containing a two-component polyurethane system as binder leads to a particularly favorable composite system.
Accordingly, the priming lacquer used in the process according to the invention, which is described in detail hereinafter, performs three functions, namely:
1. it improves the adhesion of the organic coating to glass; PA1 2. it catalyzes the hardening reaction of the organic coating; and PA1 3. it improves the resistance to shattering for which the coating is intended. PA1 1. there is no need for the silane layers to be dried at elevated temperature, thereby saving energy; PA1 2. the mechanical properties of the film are better adapted to the specific requirements of shatterproofing, thereby providing for greater latitude in the formulation of the coating composition; and PA1 3. the processing time of the surface lacquer is lengthened by a factor of 15 or more which provides for easier handling and greater production safety. PA1 (a) as binder a polyurethane polyurea which has a linear molecular structure and which is soluble in lacquer solvents and, in addition, PA1 (b) at least one catalyst which accelerates the isocyanate addition reaction leading to hardening of the coating composition. PA1 (a) a suitable solvent or solvent mixture, PA1 (b) an elastomeric polyurethane polyurea which has a substantially linear molecular structure, which is soluble in the solvent, and which is used in a quantity of from about 5 to 25% by weight, based on the total weight of the priming lacquer, PA1 (c) a silane adhesion promoter which is generally used in a quantity of from about 0.2 to 3% by weight, based on the total weight of the priming lacquer and PA1 (d) at least one catalyst which accelerates the isocyanate addition reaction responsible for the hardening of the coating composition and which is used in a total quantity of from about 0.2 to 2% by weight, based on the total weight of the priming lacquer. PA1 R represents an alkyl radical containing from 1 to 4 carbon atoms, PA1 X represents an alkylene radical containing from 2 to 4 carbon atoms and PA1 Y represents a primary amino, hydroxyl, mercapto or ##STR1## PA1 (a) as binder a two-component system reacting fully to form a polyurethane and, optionally, PA1 (b) standard auxiliaries and additives, such as leveling agents, stabilizers or UV-absorbers.
This gives rise to the following advantages over the hitherto known procedure: